where to find how many days of inventory a car maker has?

So, it's easy to find out auto sales for recent (and sometimes not so recent) months for any automaker seling in the US, but what's not easy to find is how many days supply a maker has along w/the details for each model.

The # of days of supply are only mentioned sporadically in articles like http://news.yahoo.com/s/nm/20060703/bs_nm/auto_sales_dc_3 and http://www.forbes.com/columnists/columnist..._0424flint.html. Anyone know where to find more complete info than just random tidbits?

It seems (from other auto publications) that the # of days supply is a "better" metric of how well something is selling than just absolute #s.